The topaz demon statuette is found on a plinth near the skeletons in the Lumbridge Catacombs, which can only be entered after completing The Blood Pact. When you take the statue a level 11 skeleton will attack, but you do not need to kill it to take the statue. Currently the only use for it is to sell it to Xenia for 200 coins. If you do choose to sell it to Xenia, you will not be able to get it back from the Lumbridge Catacombs.